1. Field of the Invention
The present invention relates to a voltage selecting circuit and a voltage providing circuit utilizing this voltage selecting circuit, and more particularly, to a signal delaying system utilizing the aforementioned voltage selecting circuit and voltage providing circuit.
2. Description of the Prior Art
In a typical memory control system, a delayed lock loop (DLL) circuit is generally utilized to provide a required clock signal. The memory can be operated in a normal mode, a self refresh mode, or a power down mode. The normal mode is that the memory is normally accessed and the DLL operates normally for delaying an input signal (usually a clock signal) to generate a delayed signal. The self refresh mode is that the memory executes self refreshing without any help from an external circuit. The power down mode is that the memory is in a stand-by mode without performing any data access.
In the normal mode of the memory, the DLL circuit is provided with a DLL voltage (VDLL, it can also be viewed as a control voltage) to control delay time of the DLL circuit. In the self refresh mode or the power down mode of the memory, a stand-by voltage is provided to the DLL circuit while the DLL voltage provided to the DLL circuit is stopped. However, there exist some problems in this conventional mechanism. FIG. 1 illustrates a conventional relationship between the control voltage and standby voltage provided to a DLL circuit according to the prior art. As shown in FIG. 1, the control voltage is usually higher than the stand-by voltage. Therefore, after providing a stand-by voltage to the DLL circuit while stopping providing the DLL voltage to the DLL circuit, a specific time td is required to raise the voltage if the DLL circuit intends to go back in the normal mode, which leads to a delay in time as well as a drawback in the overall circuit system.